Carrier sense multiple access with collision avoidance (CSMA/CA) is a known network control protocol, in which a carrier sensing scheme is used to allow multiple radio transceivers to share the same frequency band for radio communications under collision avoidance control, and it may be applied to wireless LANs (local area networks). With CSMA/CA, a transceiver performs carrier sensing prior to data transmission in order to determine whether there is another user carrying out radio communication. The transceiver can start transmitting data packets only if no users are sensed. Presence or absence of other users can be determined by, for example, measuring an interference level in the environment. If there is another user detected, data transmission is retried after a random period of time.
FIG. 1 is a schematic diagram illustrating a typical CSMA/CA transceiver, in which an interference level is measured at a received signal power level measuring unit to determine availability of a data transmission channel. Since with CSMA/CA a user cannot start transmission when another user is communicating, unacceptable delay may be produced in real-time communications and streaming transmission. In addition, as the number of users increases, communication may be restricted for a long period of time depending on the user environment.
Meanwhile, a technique for spatially multiplexing independent signals on the transmission side and separating the signals from each other making use of channel differences on the receiving side is proposed. See, for example, Hiromasa Fujii, et al., “A Turbo Equalizer with Simplified MMSE Filtering for MIMO Channel Signal Transmission”, 2003 IEEE 58th Vehicular Technology Conference VTC, fall, 2003. This technique allows multiple transceivers to carry out radio communications at the same time. The two transceivers communicating with each other are obviously in sync with each other; however, the transceivers communicating parallel to each other may be out of synchronization, as illustrated in FIG. 2. When multiple transceivers perform radio communication at asynchronous timing, detection accuracy of pilot symbols (or channels) from the respective transceivers and channel estimation accuracy are degraded. To this end, it is desired for communication pairs A-A′ and B-B′ to perform synchronous transmission, as illustrated in FIG. 3. In addition, when communication group 1 including the communication pair A-A′ is approaching close to or merging with another communication group 2 including communication pair C-C′, it is desired that all the communication pairs in the communication groups 1 and 2 be in sync with each other. Bringing all the transceivers to operate in sync with each other can improve the accuracy in signal detection from a spatially multiplexed signal, as well as increase the system capacity. It is also desirable that not only frames, but also symbols be in sync with each other.
To bring multiple transceivers to operate in sync with each other, the global positioning system (GPS) may be used, or alternatively, a synchronizing signal (such as a beacon) may be transmitted from prescribed communication equipment. However, using GPS is disadvantageous indoors or when not in line-of-sight, and using a synchronizing signal requires additional expense. With the latter method, synchronization cannot be assured if the transceivers are out of the service area.
Another publication, JP 10-190562A, employs a structure shown in FIG. 4, in which a reference base station A is determined in advance, and other base stations B and C are in sync with the reference base station A to control the transmission timing of each of the transceivers. However, this technique is applicable only to radio communications between a base station and a mobile station, and cannot be applied as it is to an adhoc network. In addition, it is difficult in fact in a rapidly changing environment to select and fix a base station most suitable for the reference base station in advance from among many base stations. The synchronization described in JP 10-190562A is frame synchronization, and more precise synchronization, such as symbol synchronization, is not intended. In order to appropriately carry out signal detection (or separation) from spatially multiplexed signals, symbol synchronization is required in addition to frame synchronization.
Still another publication, JP 2004-297756A, discloses a transmission timing control technique in code division multiple access (CDMA). Synchronous transmission described in this publication is between a base station and mobile stations located under this base station, as illustrated in FIG. 5. This publication does not propose synchronization between mobile stations.